Prince William and Kate with their two elder children Prince George and Princess Charlotte. Picture: Jonathan Hayward/The Canadian Press via APSource:News Corp Australia

A town crier announces that the Duchess of Cambridge has given birth to a baby boy at St Mary’s Hospital in London on April 23, St George’s Day. Picture: Jack Taylor/Getty ImagesSource:Getty Images

It could be a day or two before the name is announced, with Kate and William likely to leave the hospital in the coming hours.

The newborn’s older siblings could first make an appearance at the Lindo Wing of St Mary’s Hospital to meet him, just as George visited Charlotte at the London hospital with his father just after she was born.

The baby is now fifth in line to the throne, bumping Prince Harry into sixth place, with Charlotte becoming the first royal girl not to be usurped in the pecking order by a younger brother.

St George’s cross is on the English flag and it forms part of the UK’s Union Flag (or Union Jack).

The day is significant for the royal family, with nominations for the Order of the Garter, the highest and oldest order of chivalry in the UK, announced on April 23.

During Royal Ascot week in June, members of the Order of the Garter attend a special ceremony at St George’s Chapel in Windsor Castle — where Meghan Markle and Prince Harry are due to be married on May 19.

St George, thought to have been a soldier in the royal army persecuted for his faith, was named protector of the royal family and the patron saint of the country in the Middle Ages.

But Kate and William already used the name George for their first child. Picture: Oli Scarff/Getty ImagesSource:Getty Images

The Duke and Duchess of Cambridge with their newborn second child Princess Charlotte. Picture: AFP Photo / Leon NealSource:AFP

On Prince George’s second birthday in 2015, the Royal Mint released a limited edition commemorative coin featuring the legend of St George and the dragon he killed on one side, and a portrait of George’s great-grandmother, the Queen, on the other.

Kensington Palace announced on Monday morning local time that Kate, 36, had given birth in the same place where she had her first two children.

“Her Royal Highness The Duchess of Cambridge was safely delivered of a son at 1101hrs. The baby weighs 8 lbs 7oz [3.8kg],” it said.

“The Duke of Cambridge was present for the birth. The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news.